1. Field of Invention
This invention relates to a geophysical method and apparatus for determining the hydraulic conductivity (also known as permeability or coefficient of permeability) of porous materials.
2. Description of Related Art
The primary objective of the invention is to provide on-site/immediate hydraulic conductivity data for construction material which is used for construction quality control and quality assurance (QC/QA), as well as field documentation for submittal to a regulating authority.
Federal and/or state environmental regulations require a QC/QA program be implemented during the construction of landfills, waste disposal facilities, and certain industrial process components that are built to serve as both process containment units and waste containment systems for environmental protection. The QC/QA program involves on-site technical or engineering staff that continuously monitor construction activities and prepare certified engineering reports as to the quality of the facility construction compared to the facility design.
The construction period for building landfills, waste disposal facilities, and certain industrial process components can range from several days to several months for the initial construction, and is often an on-going process throughout the life of a facility, as is the case in construction of new waste disposal cells at a landfill.
Clay based material liners (porous material) are used in the mining, solid waste, and construction industries as impervious fluid containment barriers for the horizontal transport of process fluids or the containment of pollutants for the protection of groundwater resources. Clay based material capping systems are routinely used in waste containment or disposal systems to prevent precipitation from infiltrating into a waste containment cell, this precludes the development of leachate contaminants that could migrate downward and impact groundwater resources.
Clay liners are normally constructed to very prescriptive hydraulic conductivity design specifications as dictated by engineering requirements or state/federal regulations. An example of a regulation that calls for a prescriptive clay liner specification is the Nevada Regulations Governing Design, Construction, Operation and Closure of Mining Operations; NAC 445.242 through 445.24388. These regulations contain a minimum design criteria for the construction of leach pads and other non-impounding surfaces designed to contain and promote horizontal flow of process fluids. The regulation states:
"Containment of process fluids must consist of an engineered liner system which provides containment equal to or greater than that provided by a synthetic liner placed on top of a prepared subbase of 12 inches of native, or imported or amended soil, which has a maximum recompacted in place coefficient of permeability: of 1.times.10.sup.-6 cm/sec; or . . . " PA1 1. The testing times are generally long, requiring several days to months to complete. PA1 2. With the exception of a large scale SDRI, the tests generally assess a very small volume of compacted clay liner material, less than one cubic foot. This sample size is too small to evaluate the secondary features (preferential flow paths) of the test site. Not examining macro defects is a major disadvantage. PA1 3. Some of the testing procedures require a relatively thick test zone, particularly the Boutwell Permeameter. PA1 4. The testing programs are expensive, and will typically cost between $10,000 and 30,000 (e.g. one SDRI setup generally cost $25,000). PA1 5. The testing procedures are highly specialized and require a highly skilled technician for setup and monitoring over the test period. PA1 6. The tests are usually performed on a test pad, and therefore do not measure the actual hydraulic conductivity of the installed liner or component part of a composite liner system. PA1 7. The tests are fixed assembly and the measurements are made at a unique fixed site over a single period of time. There is no ability to repeat the test once the test has been completed and the assembly has been removed. PA1 8. BAT Test, the Boutwell Permeameter, and the combination field/lab testing procedure are destructive testing procedures. PA1 Archie, G. E., The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics, Transaction of the American Institute of Mining and Metallurgical Engineers, Vol. 146, 1942 PA1 Schlumberger C., and Schlumberger M.; Depth of investigation attainable by potential methods of electrical exploration . . . ; AIME Technical Publication No. 315; 1930 PA1 Schlumberger C., Schlumberger M., Leonardon E. D.; Electrical Coriing: a Method of Determining Bottom-hole data by Electrical Measurements.; Transactions of the AIME; Technical Publication No. 462; 1932 PA1 Vingoe, P., Electrical Resistivity Surveying, ABEM Geophysics & Electronics, Geophysical Memorandum 5/72; 1972 PA1 Wyllie, M. R. J. and Rose, Walter D., Some theoretical Considerations Related to the Quantitative Evaluation of the Physical characteristics of Reservoir Rock from Electrical Log Data, Gulf Research and Development Co., AIME Petroleum Branch; 1949 PA1 Chapman et al., U.S. Pat. No.: 4,703,279, Method of Interpreting Impedance Distribution of an Earth Formation Penetrated by a Borehole Using Precursor Data Provided by Moving Logging Array Having a Single Continuously Emitting Current Electrode and a Multiplicity of Potential Electrodes, October 1987 PA1 1. Takes a fraction of the time to perform and calculate the results. 10 minutes to a half an hour compared to several days, weeks or months; PA1 2. measures a sufficiently large volume of material, so that macro defects will be investigated; PA1 3. is capable of assessing any given thickness of either installed or natural in-situ clay based material; PA1 4. generally costs several times less than the current tests cost; PA1 5. is capable of being performed by a technician with a moderate skill level, as opposed to a highly skilled technician; PA1 6. is performed on the actual facility component being installed; PA1 7. has the capacity to be repeated as many times as desired to investigate conditional changes or stability in the unit being tested; and, PA1 8. is non-destructive.
The soil liners or infiltration barriers normally use clay based material, consisting of high clay content soil that have physical characteristics that enable standard construction techniques for the placement of the material to meet the design specification and operational permeability coefficient criteria. Conventional testing of clay liners involves the physical measurement of fluids migration into an in-place liner or through a lab sample.
Field measurement techniques, such as a sealed single, or double, ring infiltrometer, generally takes several days, weeks or even months to complete a test which causes construction and project development delays. Field measurements are typically conducted on pre-construction test pads that are built to assess the construction material and construction techniques prior to actual construction. The hydraulic conductivity test data are correlated with other material specifications including material gradation, plastic limit, Atterberg limit, density, and moisture content. The relationship between the hydraulic conductivity and the other tests is then used to develop a construction quality assurance/quality control program. The construction is generally then monitored and managed with the use of geotechnical tests, such as nuclear density and moisture content of the soil during construction.
Existing technologies for measuring hydraulic conductivity of a porous medium:
The state-of-the art methods for field investigation of clay-based liners or caps for waste disposal facilities includes the Sealed single ring infiltrometer (SSRI), and the Sealed double ring infiltrometer (SDRI). Other tests that are currently being used and are in the process of establishing American Society for Testing and Materials (ASTM) standards are borehole tests which include the BAT Test and the Boutwell Permeameter. A combination field sampling and lab testing program is also routinely used to assess the hydraulic conductivity character of a clay-based soil liner that is under construction. All of the current tests measure the direct flow of fluids through either a test pad at a facility or the installed liner at the facility. The existing technologies do not use geophysical methods as a part of the operations and calculations. The existing field technologies are described below.
Sealed single ring infiltrometer (SSRI): Tests are typically performed as construction quality assurance test prior to or during the installation of a waste disposal clay-based liner or caps, or component parts of a liner composite system. The tests take several days, weeks or months to complete. The tests are usually performed on a test pad, and therefore do not measure the actual hydraulic conductivity of the installed liner, or component part of a composite liner system. The end result of the test is that the investigator has a data set for the test pad. The data set typically includes hydraulic conductivity of the test pad, and construction parameters such as in-place density and moisture, gradation analysis, plastic index, etc., which are then used in the QC/QA program during the actual constructed clay based liner. The tests generally assess a relatively small volume, typically less than one cubic foot. This sample size is generally to small to evaluate the secondary features preferential flow paths) of the porous material.
Sealed double ring infiltrometer (SDRI): Tests are similar to SSRI, but more sophisticated and elaborate in measuring the hydraulic conductivity of a test pad. The tests are typically performed over a larger area than the SSRI, and therefore they have a greater ability to evaluate secondary macro features. The tests generally assess a volume of three to five feet in diameter by 12 to 18 inches thick. This sample size is thought to be large enough to evaluate the secondary features (preferential flow paths) of the porous material, thereby examining macro defects.
BAT Test: This test is a destructive test that requires drilling a small borehole (typically 1 inch) into the liner that is being investigated. The volume of influence for the testing is thought to be a sphere with a 3 to 6 inch diameter. A probe is sealed in the borehole during the testing and water is extruded into the surrounding environment. The rate of flow and the pressure is measured and the hydraulic conductivity is calculated for the porous material at the discreet test site. The tests are comparably inexpensive with respect to contemporary techniques, and take several minutes to a few days.
Boutwell Permeameter, also referred to as the Two Stage Borehole Permeameter: This test is a destructive test that requires grouting the testing apparatus into a borehole in the test site. A testing construction quality assurance program usually involves a series of tests to evaluate the overall permeability characteristics of a liner system.
Field/Lab Combination Testing: In addition to existing field technologies that are available to assess the hydraulic conductivity, a combination field/lab testing program is sometimes used to determine the permeability characteristics of a given construction project. In general, the procedures require obtaining a sample of the material that will be installed as the clay liner and then performing lab tests on the sample. The samples are collected in one of two ways. The preferred sample collecting method involves driving a two inch, or a two and one half inch diameter, six inch long brass tube into the test pad, or construction material borrow source and then performing the tests on the "undisturbed" sample. The term undisturbed is however misleading because the soil sample is dedensified during the collection process.
A second method of obtaining a clay liner sample is to measure the density of the in-situ soil, then collect the sample by digging enough soil product out of a hole in the vicinity of the density test, followed by remolding the soil product into a cylinder in the lab prior to performing the permeability test. Lab tests that are performed on the sample include either a "Rigid Wall" or a "Flexible Wall". The tests typically take three to five days to perform, once the sample arrives at the lab. The turn-around time from sample collection, and lab testing, to useful analytical results is typically five to seven days. The accuracy of this test procedure is thought to be in the order of plus or minus a full magnitude (i.e. .+-.1.times.10.sup.-1 cm/sec). The tests cost between $250 and $300 per each.
Test limitations for the above mentioned techniques are as follows:
The state-of-the-art technologies that are in use today have very little in common with the present invention. The existing technologies utilize physical fluid measurement techniques to determine the migration rate of fluids through soil products. The tests are generally slow, destructive, cumbersome, expensive, and often inaccurate.
Development of a geophysical method and apparatus for determining the hydraulic conductivity of porous materials at the earths surface utilizes three primary principles of applied geophysics. All three of the geophysical principals had their origin in the petroleum industry and were not considered, assessed, examined, or adapted for use the geotechnical engineering until Anderson and Ehni recognized their potential, conducted research to assess adaption of the principals, and developed the invention that is presented herein.
The first geophysical principal is based on work by Conrad and Marcel Schlumberger (1930) who developed a system of measuring the resistivity of surface rocks with electrodes deployed on the surface. The technique ultimately evolved into an expanding electrode array with several resistivity measurements at various electrode spacings. The electrode separation for a typical Schlumberger sounding will usually cover several measurements over a span of several hundred feet. The objective of the investigation is to measure the resistivity of several layers of rock formations in the subsurface. The variations in resistivity was used to interpret the subsurface geology. They later applied this technology to evaluating well bores drilled for petroleum exploration.
The second geophysical principal uses G. E. Archie's 1941 work. Archie presented his work in 1942 in a paper entitled The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics. Archie determined porosities of various materials using resistivity measurements. Mathematical formulas that G. E. Archie derived, and other relevant mathematical formulas that have been adapted for use in the invention, are outlined in the Description of the Preferred Embodiments.
The third geophysical theory expands on the permeability research developed by Tixier (1949) and Wyllie and Rose (1950). They recognized that permeability is directly related to porosity and calculated permeabilities of subsurface formations derived from resistivity data made from well bore measurements. Their work was specific to reservoir rock analysis through borehole testing in the petroleum industry.
By combining these three principles, which were directed toward the petroleum exploration and production industries, an accurate method has been developed for determining the hydraulic conductivity of porous materials at the near surface.
Earlier researchers never provided a process or method for surface investigations of hydraulic conductivity because their focus was directed towards identifying and assessing highly permeable petroleum reservoir rock that were typically sandstone formations at considerable depth (i.e. 3000 to 10,000 feet deep).
The technique developed by Anderson and Ehni employs a relatively small separation for the electrodes separation (i.e. 4 to 9 cementers). The objective of Anderson and Ehni's initial work was to obtain a single resistivity value for a relatively thin homogeneous material. This stand alone resistivity number is then used to calculate the hydraulic conductivity of the investigated surface zone. Anderson and Ehni did not test for changes in resistivity that represent changes in geologic formations. This premise, of using a unprecedented short electrode spacing for surface investigation, enabled the measurement of single layers of soil products (i.e. clay or soil layers).
Permeability calculations developed by Wyllie and Rose in 1950, or Tixier in 1949, use resistivity measurements and porosity calculations as developed by G. E. Archie, combined with a formation factor or constant. These formation and/or solution factors were empirically derived through experimentation and testing for repeatability.
The following prior art reflects the state of art of which the applicant is aware and is included herewith to discharge applicant's duty to disclose relevant prior art. However, that none of these references teach singly nor render obvious when considered in any conceivable combination the nexus of the instant invention as disclosed in greater detail hereinafter and as particularly claimed.
The present invention provides a solution to an existing, unsolved problem. The geotechnical problem involves efficient and accurate measurement of hydraulic conductivity of clay-based soil liner as used in the mining and industrial process facilities, and waste disposal and/or containment facilities. The present invention solves this geotechnical engineering problem by uniquely combing several previously uncombined technologies from the totally unrelated and non-analogous art of oil well production and oil reservoir analysis.
Those skilled in the art of geophysical borehole testing never before addressed the unrelated field of geotechnical engineering aspects of assessing hydraulic conductivity of materials on the earths surface. Even if those skilled in the art of geophysical borehole testing tried to combine the non-obvious multiplicity of geophysical steps that were combined by D. M. Anderson and W. J. Ehni, they would have failed for several reasons. Research and Development by D. M. Anderson and W. J. Ehni provided a knowledge that the resistivity of the permanent was a very important key to assessing the hydraulic conductivity of a clay-based soil liner. In addition, the mathematical constants that are used in the equations are derived from the field of geotechnical-soils engineering.
A second important step, that would be non-obvious to those skilled in the art of geophysical borehole testing, includes the use of existing geotechnical engineering techniques as a means of determining the formation factors that are needed to calculate the clay-based soil hydraulic conductivity. The multiplicity of steps that are combined and modified to yield the end result took the recognition that an existing problem, followed by research and development to employ unrelated technologies in a non-obvious manner.
Development of new geotechnical engineering, i.e. a geophysical method and apparatus for determining the hydraulic conductivity of porous materials utilizes three primary principles of applied oil field geophysics. By uniquely modifying and combining three geophysical principles that were directed primarily toward the petroleum exploration industry in the 1930's through the early 1950's, an accurate method has been developed for determining the hydraulic conductivity of porous materials at the near surface.